1. Field of the Invention
The present invention relates to a Raman amplifier for amplifying a signal light in an optical communication system. More particularly, the present invention relates to a Raman amplifier for amplifying wavelength division multiplexed signal lights.
2. Description of the Related Art
Almost all optical amplifiers used in current optical communication systems are rare-earth doped optical fiber amplifiers. Particularly, erbium (Er) doped optical fiber amplifiers (EDFA) are commonly used.
Moreover, with wavelength division multiplexing (WDM) optical communication systems, a plurality of signal lights at different wavelengths are multiplexed together and then transmitted through a single optical fiber. Since an EDFA has a relatively wide gain band, WDM optical communication systems use EDFAs to amplify the multiplexed signal lights. Therefore, with WDM optical communication systems using EDFAs, the transmission capacity of an optical fiber can be greatly increased.
Such WDM optical communication systems using EDFAs are extremely cost effective, since they can be applied to previously laid optical fiber transmission line to greatly increase the transmission capacity of the optical fiber transmission line. Moreover, an optical fiber transmission lines has virtually no limitation on bit rate since EDFAs can easily be upgraded in the future, as developments in optical amplifier technology occur.
Transmission loss of an optical fiber transmission line is small (about 0.3 dB/km or less) in the wavelength band of 1450 nm to 1650 nm, but the practical amplifying wavelength band of an EDFA is in a range of 1530 nm to 1610 nm. Thus, an EDFA is only effective for amplifying signals in a portion of the wavelength band of 1450 nm to 1650 mn.
In a WDM optical communication system, a predetermined transmission characteristic may be obtained by suppressing fluctuation of optical power among each channel to 1 dB or less in each optical repeating stage because the upper limit of optical power is caused by a non-linear effect and the lower limit by a receiving signal-to-noise ratio (SNR).
Here, a transmission loss wavelength characteristic of the transmission line and a dispersion compensation fiber or the like forming the WDM optical communication system must be reduced.
In a WDM optical communication system, the wavelength characteristic of transmission loss in a transmission line due to the induced Raman scattering provides the maximum influence on the wavelength characteristic of the signal light.
A key component of current WDM transmission systems is an EDFA that can amplify wavelength division multiplexed signal lights at the same time. For further improvement, such as increase of transmission capacity and realization of ultra-long distance transmission, it would be desirable to provide an optical amplifier which can amplify different wavelength bands than a conventional EDFA, while also providing the favorable characteristics of an EDFA.
In view of expanding the wavelength band of an optical amplifier to increase the transmission capacity of optical fibers, attention is being directed to a Raman amplifier.
A Raman amplifier can amplify the Stokes-shifted frequency that is shifted as much as the Raman shift of the amplifying medium from the frequency of a pump light. Therefore, a signal light can be amplified at a desired frequency with a pump light source producing a pump light of a desired wavelength.